A powerful earthquake rocked through Indonesia's district of Bantul early Saturday
morning, leaving thousands dead and hundreds of thousands homeless, in the worst
natural disaster to strike the country since the undersea earthquake that triggered
the December 2004 Sumatra tsunami. The quake could also spell trouble for nearby
Mount Merapi volcano, now on high alert for eruption.

The epicenter of the 6.3-magnitude earthquake was just southwest of Java's
capital Yogyakarta, where volcanologists at the Merapi Volcano
Observatory have been keeping close watch on the volcano. Hundreds of volunteers
from the Indonesian Red Cross, already on alert in case of a full-scale eruption
at Merapi, were reassigned to assist the earthquake's victims.

The earthquake's death toll is now near 5,700 people, with thousands more injured
and 200,000 left homeless, according to a May 30 report by Agence
France-Presse. The affected zone includes hundreds of square miles of farming
communities south of Yogyakarta, with power and telephone service out across
much of the region, the Associated Press reported May 29.
Since the quake, more than 500 aftershocks have kept displaced villagers, volunteers
and officials on edge. On Tuesday, a second, 6.0-magnitude earthquake sent Indonesia's
easternmost province of Papua trembling. No casualties from that quake, however,
have yet been reported, according to Agence France-Presse.

The earthquake occurred where the north-moving Australian plate is sliding
under the Sunda plate. Although the magnitude of Saturday's quake is much smaller
than the magnitude-9 Sumatra quake that triggered the 2004 tsunami, its epicenter
appears to have been relatively shallow, lying within the overlying Sunda plate
rather than deeper, where the two plates meet, says Paul Earle, a seismologist
at the U.S. Geological Survey's (USGS) Earthquake Hazards Program
in Denver, Colo. "Because it was in the crust, though it was much smaller,
you can have a lot of shaking near the surface," he says.

Some scientists fear that the tectonic activity also might trigger a collapse
of the lava dome at Merapi's summit, which could send searing hot clouds of
ash and steam racing down the volcano's sides and into the populated regions
at its base. Indonesian volcanologists raised the volcano alert to the highest
level two weeks ago, calling for mandatory evacuations of the nearby villages.
The volcano has yet to fully erupt, though it has been spewing lava and sending
smaller ash flows down its slopes for weeks, and many villagers have already
returned to tend their farms and livestock.

The tectonic activity could cause magma pressure inside the volcano to rise
further, possibly "switching it on," Gede Suwantika, head of the quake
monitoring section of the volcanology office in Yogyakarta, told Agence
France-Presse. Indeed, seismologists monitoring Merapi have already noted
a higher frequency of heat clouds at the volcano since the quake, he said.

"It's really hard to define, because the earthquake occurred close enough
to the volcano that there may be some kind of causal link between the two,"
says Jim Dewey, also at the USGS Earthquake Hazards Program. "There have
been cases where eruptions were preceded by earthquakes." However, he says,
"one could also imagine a tectonic situation where the injection of magma
causing the eruption would lead to an earthquake occurring. Historically, there's
just no clear pattern that enables us to sort that out."

Meanwhile, rescue workers have poured into Bantul to search through the rubble
for survivors and offer medical assistance, as thousands of patients crowd nearby
hospitals and doctors face a shortage of medical supplies. An international
aid effort comprising at least 22 countries, according to the United Nations,
is under way to provide both critical relief supplies and aid workers.